#if, #elif Preprocessor Directives

The #if and #elif directives compare the value of the expression to zero.

If the constant expression evaluates to a nonzero value, the tokens that immediately follow the condition are passed on to the compiler.

If the expression evaluates to zero and the conditional compilation directive contains a preprocessor #elif directive, the source text located between the #elif and the next #elif or #else preprocessor directive is selected by the preprocessor to be passed on to the compiler. The #elif directive cannot appear after the preprocessor #else directive.

All macros are expanded, any defined() expressions are processed and all remaining identifiers are replaced with the token 0.

The expressions that are tested must be integer constant expressions with the following properties:

No casts are performed.
Arithmetic is performed using long int values.
The expression can contain defined macros. No other identifiers can appear in the expression.
The constant expression can contain the unary operator defined. This operator can be used only with the preprocessor keyword #if. The following expressions evaluate to 1 if the identifier is defined in the preprocessor, otherwise to 0:
```
    defined identifier
    defined(identifier)
```
For example:
```
    #if defined(TEST1) || defined(TEST2)
```

Note: If a macro is not defined, a value of 0 (zero) is assigned to it. In the following example, TEST must be a macro identifier:

#if TEST >= 1
    printf("i = %d\n", i);
    printf("array[i] = %d\n", array[i]);
#elif TEST <0
    printf("array subscript out of bounds \n");
#endif

Conditional Compilation Directives
Preprocessor Directives

#else Preprocessor Directive
#endif Preprocessor Directive
#ifdef Preprocessor Directive
#ifndef Preprocessor Directive
List of Preprocessor Directives